This invention relates to a symbol indication drum adapted for use with apparatus requiring symbol indication, e.g. time measuring instruments or computers.
Symbols may be provided on the outer peripheral surface of a symbol indication drum directly by superposing them thereon by printing or etching, or indirectly by affixing to a drum body a belt on which symbols are formed beforehand. In either case, it is not possible to give accurate information by symbol indication unless the symbols are all disposed in predetermined positions with a high degree of precision.
When the symbols are superposed on the drum by printing, for example, it often happens that the master plate used slips and causes the symbols to be inordinately extended longitudinally or displaced transversely, with a result that the symbols are not correctly spaced apart from one another. When a symbol carrying belt is affixed to the drum body, expansion or contraction of the belt may be uneven throughout the length of the belt, thereby causing the symbols to be disposed at irregular intervals.
In testing whether or not the symbols are disposed in correct positions when a symbol carrying belt is affixed to a drum, there has hitherto been used a cylindrical tester sleeve having a transparent window, such as shown in FIG. 2, with a frame of reference lines denoting the height and width of the drum indicia. However, use of a tester sleeve such as shown in FIG. 2 has been time-consuming, as each drum indicia must be individually checked for both lateral and longitudinal positioning, which has suggested that the production of symbol indication drums can be facilitated by improvements in the drum as well as in the means for testing or checking the position of the indicia thereon.